The present invention relates to a power supply device that precharges a smoothing capacitor arranged in a load when starting to supply the load with power, and more particularly, to an improved detection structure that increases the accuracy for determining completion of precharging.
An electric vehicle, which uses a motor as a drive source, typically converts high voltage direct current (DC) power to alternating current (AC) power with an inverter to drive a motor. A DC power supply, which is formed by a battery pack, supplies the high voltage DC power. Normally, an electrolytic capacitor, which has a large capacitance, is arranged between input terminals of the inverter to smooth voltage fluctuations in the supplied DC power. However, when the supply of power is started, the arrangement of such a smoothing capacitor results in a problem. When the supply of power to the inverter is started in a state in which the smoothing capacitor is not sufficiently charged, the smoothing capacitor is rapidly charged immediately after the supply of power starts. As a result, a large amount of current flows to a main contactor, which is connected to the inverter to supply power. This may fuse the contacts of the main contactor.
Therefore, a power supply device normally precharges the smoothing capacitor before starting to supply power to the inverter. FIG. 1 shows the circuit configuration of a power supply device that precharges a smoothing capacitor. The power supply device shown in FIG. 1 includes a battery pack 50, which is a DC power supply. The battery pack 50 has a positive electrode (+) and a negative electrode (−) connected via a positive main contactor 51 and a negative main contactor 52 to a positive input terminal and a negative input terminal of an inverter 53, respectively. A large capacitance electrolytic capacitor, or smoothing capacitor 54, is connected between the two input terminals of the inverter 53. A precharge contactor 55 is connected in parallel to the positive main contactor 51. Further, a precharge resistor 56 is connected in series to the precharge contactor 55 to restrict the flow of current during precharging.
In such a power supply device, the starting of power supply to the inverter 53 is controlled in the following manner. First, the negative main contactor 52 is closed. Then, the precharge contactor 55 is closed. This starts the precharging of the capacitor 54. In this state, the precharge resistor 56 restricts the flow of current. Thus, the capacitor 54 is gradually charged. When it is determined that the precharging has been completed, the positive main contactor 51 is closed to start supplying power to the inverter 53.
Normally, the completion of precharging is determined by detecting the voltage between the terminals of the capacitor 54 with a voltmeter. Referring to FIG. 2, a curve L indicates the shifting of the voltage between the terminals of the capacitor 54 (precharge voltage Vpc). As shown by curve L, after precharging starts at time t0, the precharge voltage Vpc gradually increases from 0 V and ultimately saturates at a certain voltage (saturation voltage Vst). The saturation voltage Vst is equal to the power supply voltage of the battery pack 50 subsequent to the voltage decrease caused by the precharge resistor 56. Accordingly, a voltage determination value Vref, which is slightly lower than the saturation voltage Vst, is set to determine precharging completion when the precharge voltage Vpc exceeds the voltage determination value Vref.
However, the determination of the precharging completion based on the precharge voltage Vpc has a shortcoming in which the power supply voltage of the battery pack 50 may increase or decrease drastically due to various conditions, such as wear, individual differences, and the charge level. In such a case, the saturation voltage would vary accordingly. Thus, when set at a fixed value that is constant, the voltage determination value Vref would not be applicable for such changes in the power supply voltage. As a result, the precharging completion may not be properly determined. For example, as shown by curve L′ in FIG. 2, when the power supply voltage of the battery pack 50 is low, the precharge voltage Vpc would saturate at a saturation voltage Vst′, which corresponds to such a low power supply voltage state and which is lower than the saturation voltage Vst. Therefore, even if precharging is actually completed, the precharge voltage Vpc does not reach the voltage determination value Vref. In such a case, precharging completion would not be determined no matter how much time elapses. On the other hand, as shown by curve L″, when the power supply voltage of the battery pack 50 is high, the precharge voltage Vpc would reach the voltage determination value Vref before reaching a saturation voltage Vst″, which corresponds to such a high power supply voltage state. Therefore, the supply of power to the inverter 53 would be started in a state in which precharging is still insufficient. As a result, a large current may fuse the contacts of the positive main contactor 51. In such a manner, when the voltage determination value Vref is constantly fixed, the precharging completion determination, which is based on the precharge voltage Vpc, may be inaccurate when the power supply voltage varies.
Accordingly, in the prior art, Japanese Laid-Open Patent Publication No. 10-304501 describes a scheme for determining precharging completion from a precharge current Ipc. In this scheme, the current that flows from the battery pack 50 to the smoothing capacitor 54 during precharging, that is, the precharge current Ipc, is used to determine precharging completion. Referring to FIG. 3, the precharge current Ipc gradually decreases as the smoothing capacitor 54 becomes charged and ultimately saturates at 0 A. The saturation value 0 A of the precharge current is constant and fixed regardless of the power supply voltage of the battery pack 50. Thus, in the scheme described in the above publication, a current determination value Iref, which is slightly higher than 0 A, is set to determine precharging completion when the precharge current Ipc becomes less than the current determination value Iref.
A change in the power supply voltage would not affect the precharging completion determination when using the precharge current Ipc. However, the actual determination accuracy would become insufficient due to detection errors of the precharge current Ipc. More specifically, the current supplied to the smoothing capacitor 54 during precharging is restricted by the precharge resistor 56 and thus small. The current becomes even smaller as the smoothing capacitor 54 becomes charged. As a result, even if relatively small noise is superimposed on the precharge current Ipc, this would significantly affect the detection accuracy. In this manner, the precharging completion determination would be easily affected by noise. Thus, the accuracy detection of the precharge current Ipc, which has a low S/N ratio, would become insufficient. This would result in insufficient accuracy for the detection of precharging completion. Hence, to accurately perform precharging completion determination based on the precharge current Ipc, sufficient noise measures must be taken. Otherwise, an ammeter having a high resolution must be used. In such cases, the manufacturing costs would increase.
Such a problem is not limited to a power supply device that is used with an inverter and is common in any power supply device that supplies DC power to a load including a large capacitance smoothing capacitor and precharges a smoothing capacitor when starting the supply of power. Therefore, for a power supply device of which power supplying subject is a load including a smoothing capacitor that undergoes precharging, it is desired that precharge completion be accurately determined without increasing manufacturing costs regardless of the type of load.